Interface Inferred from Property Not Rejecting Invalid Properties

[MichaelPastuch]

Bug Report

🔎 Search Terms

"type infer", "conditional interface"

🕗 Version & Regression Information

• This is the behavior in every version I tried (4.2, 4.6, 4.8, and nightly)

⏯ Playground Link

Playground link with relevant code

💻 Code

type TNumTypes = 1 | 2 | 3;

interface IBase {
	type: TNumTypes;
}

interface IString extends IBase {
	type: 2;
	string: string;
}

interface IDate extends IBase {
	type: 3;
	date: Date;
}

const test: IBase | IString | IDate = {
	type: 3,
	string: "test"
};

🙁 Actual behavior

The test object with a type value of 2 should flag the string property as invalid, since it is not in IBase or IDate, but the object is considered valid.

🙂 Expected behavior

The test object should infer its interface from the value of type. Any valid value could be an IBase, but a value of 2 should not be an IDate and a value of 3 should not be an IString.

[MartinJohns]

Duplicate of #20863. Your object is a valid IBase. Additional properties are allowed. Objects are not sealed.

You should instead create a discriminated union type.

[fatcerberus]

IIRC this is a limitation of excess property checking; if the property exists in any of the union constituents, it isn't flagged. Note that excess property errors are not type errors per se; TypeScript doesn't support exact types.

[MichaelPastuch]

Thank you for the swift responses. I am happy with the explanation and can work around my issue.